If i was forced to decide for just one it would be Super Mario Galaxy 2. Donkey Kong Country Returns, Punch-Out!!, House of the Dead: Overkill, Rock Band 2 and Metroid Prime 3: Corruption come very close though. Still have to play quite a few Wii games so that might chnage in the future.

For me, it's Mario Kart Wii and Super Smash Bros. Brawl. I've only played no more than five Wii games though, so that's why. xD Although I really think those two would be at the top of the list regardless.